Simplified... (oops concentration probs edited for math revesal and typed S.D. instead of B.C.)Sectional Density = mass/diamerter2 So for any thing less than about a 50%-60% meplat at subsonic speed a decent estimate of B.C. will be close to it's S.D.slightly higher for longer pointy noses...slightly lower for large meplats... seems to hold true for the g1 drag curve... other models well have not thought about it much...So, 70g .257 B.C. is about .151 ; 85g .257 B.C. is about .184 should be close enough for a ballpark figure...oh the above is mass in pounds not grains... so to use grains it is mass in grains /7000...